This is a little hard to explain without showing in person...if nobody understands, I'll try to post a video.

The question is when slapping, should I be rotating my wrist on a flat horizontal plane, so much so that my thumb stays in perfect line with the string? Or can I have some rotation away from the plane of the string, alsmost at an angle, and then come down on the string sort of diagonally?

I don't really think this is a topic covered in the Slap Bass thread.

Whatever works for you. I prefer to play the way Marcus Miller plays with his thumb lined up along the string horizontally. I believe Flea plays with that diagonal style as do many others.

+1 on marcus miller style.
When you get a bit more experienced, you might wanna try out the double-thumb technique, and you can't do that playing like flea does, so save yourself the trouble and play Miller-style from the beginning

I was taught the 'flea' way, unfortunately i can't double thump duye to this, re teching ymself something i've done for 5 years isn't going to be easy :(

I've never got how to hit just one string with the thumb using the perpendicular (diagonal) approach. Keeping the thumb parallel to the strings works best for me.

Find out what works best for you. I like keeping my pinky knuckle almost always touching the bass at all times.

i slap the diagonal way, but i still can double thump pretty well. Even interchangeably. All i do is shift my wrist a little bit.

I slap diagonally, but I'd like to learn to slap with the thumb more parallel to the strings to eventually learn double-thumbing. It just feels awkward to me as I need to be more careful not to touch the string with the root of my thumb and I can't really find a good place to anchor my hand....

Do what feels right.What will work comfortably is going to depend on bass position. If you have it round your knees it will be alot more natural to play horizontal, if you have it round your throat diagonal will feel more natural (Note I am not recommending either of the above, just noting extremes)

I slap from all angles depending on what tone i want. I think its important to be able to do all your techniques in different ways
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When I slap the E string I smack it diagonally. But to hit the other strings, I used to mute with my left hand which left my popping slow and sloppy.

I am now working on the parallel slapping...and the double thumb/index/middle combo rocks...can only do it at 40bpm at the moment, but it's developing sloooooowly :)